Atomic layer deposition of selected molecular clusters

ABSTRACT

Energy bands of a thin film containing molecular clusters are tuned by controlling the size and the charge of the clusters during thin film deposition. Using atomic layer deposition, an ionic cluster film is formed in the gate region of a nanometer-scale transistor to adjust the threshold voltage, and a neutral cluster film is formed in the source and drain regions to adjust contact resistance. A work function semiconductor material such as a silver bromide or a lanthanum oxide is deposited so as to include clusters of different sizes such as dimers, trimers, and tetramers, formed from isolated monomers. A type of Atomic Layer Deposition system is used to deposit on semiconductor wafers molecular clusters to form thin film junctions having selected energy gaps. A beam of ions contains different ionic clusters which are then selected for deposition by passing the beam through a filter in which different apertures select clusters based on size and orientation.

RELATED APPLICATION

This patent application claims benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 61/867,930, filed on Aug. 20, 2013,which is hereby incorporated by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure relates to the fabrication of nanometer-sizedintegrated circuit FET (field effect transistor) devices and, inparticular, to methods of tuning performance of the FETs byincorporating selected molecular clusters.

Description of the Related Art

As technology nodes for transistors scale below 10 nm, maintainingcontrol of various electrical characteristics in bulk semiconductordevices becomes increasingly more challenging. Such electricalcharacteristics include, for example, transistor threshold voltage andcontact resistance. The threshold voltage of a transistor fundamentallygoverns the transition from an “off” state to an “on” state, andtherefore dictates the switching speed and the off-state leakage currentof the transistor. By tuning the threshold voltage of a transistor,integrated circuit designers can optimize transistor performance bybalancing the need for fast switching speed with the need for low powerconsumption. For example, circuit designers may choose to place lowthreshold voltage (LVT) transistors, which are fast but leaky,specifically in critical circuit paths that have maximum delays. Slower,high threshold voltage transistors that have low leakage current in theoff state can then be used in non-critical paths so that powerconsumption stays low. Thus, it is advantageous to be able to providetransistors on the same integrated circuit chip that have a range ofthreshold voltages. Contact resistance at the interface between thesource and drain terminals of the transistor and the interconnectstructure is another important factor for transistor performance.Keeping the contact resistance low increases signal transmission speedswhile reducing power dissipation.

The threshold voltage and the contact resistance of integratedtransistor devices are related to atomic, molecular, and crystallineproperties of solid state materials used to form the source, drain, andchannel regions. Thus, tuning the transistor performance generallyinvolves adjusting material properties of the source and drain regionsand of the channel region. Conventional methods of forming source anddrain regions have focused on implanting dopant ions in the substrateand annealing the implantation damage to re-crystallize the dopedregions. Doping profiles of the source and drain regions can be craftedin this way to influence the transistor threshold voltage. Gate oxidethickness and material properties have also been optimized to improvecontrol of the threshold voltage. More recently, methods have beendeveloped to increase charge carrier mobility within the channel regionby imparting tensile or compressive stress to the channel. One way tostress the channel is to alter the gate stack. Another way ofintroducing stress in the channel is to form epitaxially grown raisedsource and drain regions, or epitaxially grown layers within thechannel. As semiconductor technology nodes continue scaling down tosmaller device dimensions, satisfying the requirement to achievedifferent threshold voltages (V_(t)) for different devices becomesextremely challenging, especially at gate lengths below 10 nm.

Transistor performance parameters such as threshold voltage and contactresistance fundamentally depend on the shape of the energy bandstructures at material interfaces between p-type and n-type materialswithin the device. Such interfaces are formed at the junction of thesource region and the channel, at the junction of the drain region andthe channel, and at metal contacts to the source and drain regions. Eachsemiconducting material on either side of an interface has acharacteristic energy gap that represents the energy input needed tofree electrons from the atoms, thus making available charge to conduct acurrent. An electric potential difference at the interface is overcomeby applying a bias voltage that is equal to or greater than thethreshold voltage.

The energy band structure of a thin film material is influenced bydeposition methods and ambient conditions present during formation ofthe film. Techniques such as epitaxial growth and atomic layerdeposition (ALD) attempt to control film deposition at molecular andatomic levels. Density function theory (DFT) studies familiar to thepresent inventor predict that the energy gap of a device that includes amolecular cluster thin film in which the cluster size is less than 1 nmwill be determined by atomic orbital interactions within the molecules.Such effects of molecular clusters are described in the followingjournal papers by the present inventor, each of which is herebyincorporated by reference in its entirety: “Theoretical and ExperimentalStudies of Silver Bromide Clusters,” Doctoral dissertation by HongguangZhang, University of Texas at Arlington, 2000, hereinafter, “Zhang”;“Theoretical Study of the Molecular and Electronic Structures of NeutralSilver Bromide Clusters (AgBr) n, n=1-9,” H. Zhang, Z. A. Schelly, andD. S. Marynick, Journal of Physical Chemistry A, Jun. 10, 2000, vol.104, pp. 6287-6294, hereinafter, “Zhang, et al.”; and “Preparation ofAgBr Quantum Dots via Electroporation of Vesicles,” N. M. Correa, JohnH. Zhang, and Z. A. Schelly, Journal of the American Chemical Society,Jun. 23, 2000, vol. 122, pp. 6432-6434, hereinafter “Correa et al.” Alsoincorporated by reference in their entireties are the following relatedU.S. Patent documents to the present inventor: U.S. patent applicationSer. No. 13/931,096, entitled, “Quantum Dot Array Devices with MetalSource and Drain,” filed Jun. 28, 2013; U.S. patent application Ser. No.13/931,234, entitled “Threshold Adjustment for Quantum Dot Array Deviceswith Metal Source and Drain,” filed Jun. 28, 2013; and U.S. PatentApplication Publication No. US2013/0093289, entitled “Size-controllableOpening and Method of Making Same,” filed Apr. 18, 2013.

BRIEF SUMMARY

Energy bands of a thin film containing molecular clusters are tuned bycontrolling the size and the charge of the clusters during thin filmdeposition. Using atomic layer deposition (ALD), an ionic cluster filmis formed in the gate region of a nanometer-scale transistor to adjustthe threshold voltage (V_(t)), and a neutral cluster film is formed inthe source and drain regions to adjust contact resistance. A workfunction semiconductor material such as a silver bromide (AgBr) or alanthanum oxide (LaO_(x)) is deposited so as to include clusters ofdifferent sizes such as dimers, trimers, and tetramers, formed fromisolated monomers. Different molecular clusters have differentassociated energy band gaps depending on the atomic orbital interactionsof the constituent atoms.

A type of Atomic Layer Deposition (ALD) system is disclosed, along witha method for depositing on semiconductor wafers certain molecularclusters to form thin film junctions having selected energy gaps. In oneembodiment, an ion source produces a beam of ions containing differentionic clusters which are then selected for deposition by passing thebeam through a charge filter and a size filter. The charge filteremploys a magnetic field. The size filter has different apertures toselect for cluster size and orientation. Methods of enhancing theproduction of high spin clusters are also disclosed, as are methods ofcontrolling the sizes of clusters produced at the source. A desiredcluster film thickness and uniformity can be achieved by varying thespeed of a wafer transport mechanism supporting the target wafer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIG. 1A is a molecular diagram of a monomer having a silver atom and abromine atom.

FIG. 1B is a molecular diagram of a dimer made by combining two monomersas shown in FIG. 1A.

FIG. 1C is a pair of molecular diagrams illustrating building a trimerfrom the monomer shown in FIG. 1A and the dimer shown in FIG. 1B.

FIG. 1D is a pair of molecular diagrams illustrating building a tetramerfrom two monomers as shown in FIG. 1A and the dimer shown in FIG. 1B.

FIG. 2 is a theoretical plot of a predicted relationship between energyand cluster size for molecular clusters up to 100 Å.

FIG. 3 is an expansion of the plot shown in FIG. 2 for clusters smallerthan 10 Å.

FIG. 4A is an energy band diagram illustrating the interface of ametal-semiconductor contact.

FIG. 4B is an energy band diagram illustrating the interface of ametal-insulator-semiconductor contact.

FIG. 5 is a schematic diagram of a system for generating ionic clustersand depositing selected ionic clusters on a semiconductor wafer, basedon the electric charge and size/mass of the cluster, according to anembodiment described herein.

FIG. 6 is a magnified view of an aperture within the system shown inFIG. 5, according to an embodiment described herein.

FIG. 7 is a side view of one embodiment of the aperture shown in FIG. 6,as described herein.

FIG. 8 is a cross-sectional view of an NFET device that includesmolecular clusters at the source/drain contact interface.

FIG. 9A is a top plan view of NFET and PFET devices that have beenfabricated with source/drain quantum dots.

FIG. 9B is a cross-sectional view of the devices shown in FIG. 9A, takenalong the cut line A-A′.

FIG. 9C is a cross-sectional view of the devices shown in FIG. 9A, takenalong the cut line B-B′.

DETAILED DESCRIPTION

It will be appreciated that, although specific embodiments of thepresent disclosure are described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the present disclosure. Accordingly, the present disclosure isnot limited except as by the appended claims.

In this description, certain specific details are set forth in order toprovide a thorough understanding of various aspects of the disclosedsubject matter. However, the disclosed subject matter may be practicedwithout these specific details. In some instances, well-known structuresand methods of semiconductor processing comprising embodiments of thesubject matter disclosed herein have not been described in detail toavoid obscuring the descriptions of other aspects of the presentdisclosure.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

FIGS. 1A-1D illustrate formation of molecular clusters during thin filmdeposition. Clusters may be neutral molecular clusters, or they may beionic molecular clusters having a net charge, or a net chargedistribution. Formation of molecular clusters from isolated monomers hasbeen studied previously by the present inventor, as previously citedherein. Clusters are formed by attachment of isolated monomers tomolecules, or by accumulation of monomers into a molecular cluster asillustrated in FIGS. 1A-1D. A monomer is a basic molecular unit, forexample, two bound atoms, or three atoms joined in a triangular unit.FIG. 1A shows an example of a two-atom AgBr monomer 178 a having a largebromine atom 178 b and a small silver atom 178 c. Two such monomers 178a can bind together to form a dimer 178 d having four atoms, as shown inFIG. 1B. The dimer 178 d contains three chemical bonds along an axis 178e. Subsequently, another isolated monomer 178 a can attach to the dimer178 d to form a trimer 178 f that includes six atoms, three Ag and threeBr, as shown in FIG. 1C. Two isolated monomers 178 a can attach to thedimer 178 d to form a tetramer 178 g having eight atoms, as shown inFIG. 1D, and so on. All of the clusters shown in FIGS. 1A-1D are neutralmolecular clusters, because for every Ag atom the cluster also includesa corresponding Br atom. When the number of Ag and Br atoms is notequal, the cluster is an ionic cluster.

FIG. 2 shows a graph 180 of energy band gaps as a function of clustersize for various AgBr molecular clusters. The predicted relationshipsshown between energy gaps and cluster size are computed from ultravioletabsorption spectra of AgBr clusters, as described in the Zhang et al.reference. Consequently, when an AgBr metallic film is deposited onto asilicon substrate, the energy band gap at the silicon-metal interface isaltered depending on the AgBr cluster size. For cluster sizes in therange of about 20-100 Å, the associated energy band gap 182 of the AgBrfilm decreases monotonically from about 3.1 to 2.7 eV. However, thereexists a transition 179 below which AgBr clusters are associated withmuch larger and wider ranging energy band gaps 184. The transition 179occurs at about 20 Å. At molecular cluster sizes below 20 Å, the energyband gap 184 is the difference between the HOMO (highest occupiedmolecular orbital) and LUMO (lowest unoccupied molecular orbital)energies, known as the HOMO-LUMO gap. The HOMO-LUMO gap is qualitativelysimilar to the semiconductor energy band gap that characterizes crystalsat large length scales, e.g., 20-100 Å. In particular, AgBr clusterssmaller than about 10 Å have HOMO-LUMO gaps within the range of about3.5 eV to 5.5 eV, which values are similar to those of the energy bandgaps associated with nFET and pFET band edges.

A similar relationship is known to exist for other molecular clusterssuch as, for example, sulfur oxide. Analogously, LaO₂ clusters areexpected to exhibit similar behavior. Thus, controlling the clusterformation process during film deposition can achieve a desired energygap and a corresponding threshold voltage. Although AgBr clusters areused here as an example, other clusters having an associated HOMO-LUMOgap close to the energy gap of silicon can also be used to tuneproperties of silicon transistors. For example, copper or gold, combinedwith one of the elements from either group 7 or group 6 of the periodictable, depending on the desired electrical properties. Alternatively,aluminum, gallium, or indium may be combined with group 1 or group 7elements to form molecular clusters.

FIG. 3 shows an expanded view of the energy band gaps 184 shown in FIG.2 for cluster sizes below 10 Å, along with molecular model diagrams ofthe associated clusters. Both ionic clusters and neutral clusters areshown in FIG. 3. For example, the monomer 178 a, the dimer 178 d, thetrimer 178 f, and the tetramer 178 g, all of which are neutral clusters,are shown next to their associated energy gap values. Ionic clusters 188a having three atoms and 188 b having five atoms each include moresilver atoms than bromine atoms, and thus carry a net charge. Thedesired cluster can be selected from the graph shown in FIG. 3. Forexample, if a thin film having an energy gap of about 4.5 eV is desired,ionic clusters 188 a can be incorporated into the film.

FIG. 4A shows a typical M-S energy band diagram 190 characterizing ametal-semiconductor interface having a band gap, Φ_(n, MS) 192. Such aninterface exists at, for example, electrical contacts to the source anddrain regions where a metal material such as copper contacts an n-typeor p-type silicon material. FIG. 4B shows an M-I-S energy band diagram194 characterizing a metal-insulator-semiconductor interface having aband gap, Φ_(n, MIS) 196. Insertion of an insulator 197 in the form of aneutral cluster film between the metal and semiconductor layers shiftsthe band structure of the contact, thus lowering the barrier forconduction by about 30% compared with the band gap 192 corresponding tothe M-S device. Commensurately, the contact resistance at the interfaceof the M-I-S device is 30% lower than that of the M-S device.

Similarly, ionic cluster films can be incorporated into the gate stackto alter the threshold voltage of the transistor by adjusting the energygap at the interface between the metal gate and the gate dielectric. Forexample, a gate dielectric made of halfnium oxide (HfO₂) formed adjacentto a metal gate having a work function of 4.9 eV can be altered byincorporating an atomic oxide into the gate dielectric. For example, byincorporating tantalum oxide (TaO₂), the effective work function of thegate is lowered from 4.9 eV to 4.3 eV, thus lowering the thresholdvoltage by 0.6 V. In a conventional device, a desired work function maybe obtained by stacking multiple metal layers onto the gate. However, amulti-layer gate stack requires a metal patterning step for eachcomponent film in the gate stack. Whereas, altering the energy gap atthe metal-dielectric interface by incorporating ionic clusters does notentail the use of additional mask patterning steps.

FIG. 5 shows an exemplary molecular cluster film deposition system 230for selectively depositing ionic cluster or neutral cluster films onto atarget wafer 231, according to one exemplary embodiment. In oneembodiment, the molecular cluster film deposition system 230 is amodified atomic layer deposition (ALD) system. The molecular clusterfilm deposition system 230 includes a precursor source 232, abeam-focusing element 234, a magnet 236, a filter 238 having an aperture239, and a wafer transport device 240. The cluster film depositionsystem 230 as shown is implemented within a vacuum chamber. Theprecursor source 232 produces ions, e.g., Ag⁺Br, or ionic clusters,e.g., (AgBr)⁺, from a gas inflow 242 using, for example, an ionizingfilament 244. The ionizing filament 244 applies an electric current toionize the gas. Ions thus produced can be formed into an ion beam 246 byan ion repeller 248, an electron trap 249, an ion accelerator 250, andthe beam focusing element 234. The ion repeller 248 prevents ions fromaccumulating inside the precursor source 232. The electron trap 249extracts free electrons from the ion source so as not to neutralize theions. The ion accelerator 250 applies an electric field to extract ionsfrom the precursor source 232 and guide the ions toward the beamfocusing element 234, which focuses the trajectories of the ions intothe ion beam 246. The ion beam 246 is then directed toward the magnet236, which deflects the overall ion beam path through an angle, shown as90 degrees. Different ionic clusters within the ion beam 246 aredeflected through slightly different angles depending on theircharge-to-mass ratio, similar to the way in which a mass spectrometeroperates. Ionic clusters within the ion beam 246 then advance toward thetarget wafer 231. When the desired molecular clusters are neutralclusters, ionic clusters in the ion beam 246 may be neutralized bydepositing onto a doped film of opposite polarity at the surface of thetarget wafer 231. Prior to landing on the target wafer 231, the ionicclusters pass through the filter 238. The filter 238 may have a singleaperture 239 that allows clusters of a certain size, charge, ororientation to pass therethrough while others are blocked.Alternatively, the filter 238 may include a plurality of small aperturesthat act as a mask through which ionic clusters pass as they approachthe target wafer 231.

With reference to FIGS. 5 and 6, there are at least three differenttechniques that can be implemented in the molecular cluster filmdeposition system 230 to place the ionic clusters at a desired locationon the target wafer 231. According to a first technique, the wafertransport device 240 is programmed to move the target wafer 231 to adesired location with respect to the ion trajectories within the ionbeam 246. In this embodiment, the wafer is positioned at the desiredlocation. According to a second alternative embodiment, the filter 238through which the ionic clusters pass as they are approaching the targetwafer 231 may also be moved to a desired location. The filter 238 can bephysically shifted to different locations by appropriate stepper motorsand mask adjustment mechanisms of the type typically used for adjustingreticles when imaging semiconductor wafers, to place the filter 238 at adesired location in order to direct the desired ionic clusters to thetarget wafer 231. In such an embodiment, a cluster mask can bepositioned over a desired location on the target wafer 231 to which thesame ionic clusters are to be delivered. The ion beam 246 containing theionic clusters is applied to the entire filter, and the ionic clusterspass through the plurality of apertures, according to their size and/ororientation, in order to be delivered at substantially the same time tothe target wafer 231 at a plurality of sites. Such a technique thereforepermits a plurality of locations on the wafer, such as all the sources,all the drains, or a subset thereof, to selectively receive the ionicclusters in order to achieve the desired electrical properties asdescribed herein.

FIGS. 6 and 7 show in greater detail a second technique in which thefilter 238 selects molecular clusters according to their orientation.For example, the ionic cluster 188 a having an axis of symmetry 262 boriented parallel to the beam direction is wider than a similar ioniccluster 188 a having an axis of symmetry 262 a oriented transverse tothe beam direction. Thus, one or more apertures 239 within the filter238 can be sized narrowly enough to pass the transverse cluster havingaxis 262 a while blocking the parallel cluster having axis 262 b.Alternatively, a filter 238 having a size-controllable aperture 264 canbe used, such as the one shown in FIG. 7, and described in the '289publication. In one embodiment, the size-controllable aperture 264includes one or more piezoelectric membranes that can be adjustablydeformed by applying an electric bias voltage 266. Thus, the aperturesize can be made to increase from an initial diameter d1 to a largerdiameter d2 that will pass the parallel cluster having axis 262 b.Filtering molecular clusters based on their orientation can be used forboth ionic clusters and neutral clusters.

A third acceptable technique is to use a steering mechanism to steer theion beam 246 toward a specific location on the target wafer 231. Asexplained herein and also illustrated in FIG. 5, the magnet 236 can beused to steer the ion beam 246 to a desired path and to reach a desiredlocation. Additionally or alternatively, the target wafer 231 can beelectrically charged so as to influence trajectories of ionic clustersas they approach the wafer surface. Beam steering may be used to providefine-tune adjustment to direct desired ionic clusters to a selectedlocation on the target wafer 231. These are just three of the techniquesthat are acceptable for ensuring that the ionic clusters, or neutralclusters, are delivered to a desired location on the wafer. Othertechniques can also be used.

The thickness and uniformity of the resulting cluster film, as well asthe density of the molecular clusters at a particular wafer location,can be selectively achieved according to desired electricalcharacteristics based on timing, movement, or combinations thereof. Forexample, the target wafer 231 and/or the filter 238 can be movedslightly during application of the molecular clusters in order toprovide a thinner layer—the faster the movement, the thinner the layer,and the more spaced out the ionic clusters are from each other.Similarly, the target wafer 231 and/or the filter 238 can be heldstationary in order to build up a large quantity of molecular clustersat particular selected locations. Additionally or alternatively, thesurface of the target wafer 231 can be charged negatively to attract Agionic clusters or positively to attract Br ionic clusters.

It is noted that deposition systems other than the molecular clusterfilm deposition system 230 as described above can be used to deposit themolecular cluster films disclosed herein. Alternative delivery systemsmay employ a jet carrier gas or a spin-on-glass deposition process, forexample. The devices and methods presented herein are not dependent uponthe deposition system shown, which is one of many sets of equipment thatmay be used. Furthermore, the filter 239, having one or more apertures239, or being equipped with the size-controllable aperture 264, can alsobe a feature of such alternative delivery systems.

FIG. 8 shows a cross-sectional view 300 of an NFET transistor 302 and aPFET transistor 304 that include molecular cluster ALD-deposited films,according to a first embodiment. The exemplary transistors 302 and 304are fully depleted silicon-on-insulator (FD-SOI) type devices formed ona silicon substrate 303. However, the molecular cluster films describedherein are suitable for use in other types of CMOS devices as well, suchas, for example, ultra-thin body buried oxide (UTBB) transistors, orcombination UTBB/FD-SOI devices, known to those skilled in the art oftransistor design and fabrication. The exemplary transistors 302 and 304include epitaxial raised source and drain regions 306 and 308,respectively, which extend downward into the substrate 303 to a buriedoxide (BOX) layer 310. An isolation trench 305 filled with an insulator,e.g., SiO₂, electrically isolates the NFET transistor 302 from the PFETtransistor 304. The isolation trench 305 may include a liner 307 madeof, for example, SiN.

The NFET transistor 302 includes a channel region 312, a low-k gatedielectric 314, a multi-layer metal gate 316, a spacer 317, and aninsulating layer 325 e.g., an inter-layer dielectric. The channel region312 may be a high-mobility strained silicon channel in which a germaniumlayer is formed over the silicon and diffused to produce a SiGe layerhaving tensile stress. At the metal-semiconductor interface of sourceand drain contacts 318 with the source and drain regions 306 and 308,respectively, an insulating molecular cluster film 320 may be insertedto form an M-S interface having reduced contact resistance. Themolecular cluster film 320 is made of AgBr or TiO₂, for example. Themolecular cluster 323 as shown is a neutral cluster.

The PFET transistor 304 includes a channel region 322, a low-k gatedielectric 324, a multi-layer metal gate 326, and a spacer 327. Thechannel region 322 may be a high-mobility strained silicon channel inwhich a germanium layer is formed under the silicon and diffused toproduce a SiGe layer having compressive stress. At the interface of twolayers within the multi-layer metal gate 326, there is inserted a workfunction ionic cluster film 330 that enhances the compressive stress inthe channel region, thereby increasing mobility of the charge carriers.The work function ionic cluster film 320 is made of AgBr or LaO₂, forexample. The molecular cluster 334 as shown is an ionic cluster in whichthe number of bromine atoms is one greater than the number of silveratoms.

FIGS. 9A-9C illustrate NFET and PFET transistors that include molecularcluster ALD-deposited films, according to a second embodiment. In thesecond embodiment, the transistors include molecular cluster quantum dotfilms 350 a in the source and drain regions and ionic cluster quantumdot films 350 n and 350 p in the channel regions of the NFET and PFETdevices, respectively. A quantum dot is a general term referring to asemiconductor nanocrystal, in the range of about 10-100 atoms indiameter, that exhibits quantum mechanical properties [Wikipediaarticle, “Quantum Dot,” August 2014]. Such quantum mechanical propertiesinclude the interfacial atomic band gap behavior described above.

FIG. 9A shows a top plan view of the two transistors, in which the PFETdevice is represented in the upper panel and the NFET device isrepresented in the lower panel. FIG. 9B shows a correspondingcross-sectional view cut through the channel regions of both devicesalong a cut line A-A′. Corresponding parts of the exemplary transistorsshown in FIGS. 9B and 9C are labeled with common reference numbers as inFIG. 8, e.g., substrate 303, buried oxide 310, isolation trench 305,metal gates 316, 326, and so on. The transistors shown in FIGS. 9A-9Cdiffer structurally from the device shown in FIG. 8, at least in thatthey have recessed gates, similar to devices known in the art anddescribed in U.S. Patent Publication No. 2007/0007571. The channelregions of the NFET and PFET devices each include a quantum dot channelfilm 350 b containing molecular clusters 338 and 340, respectively. Inone embodiment, the molecular cluster 338 is a 5-atom ionic cluster madeof, for example, Ag₂Br or La₂O, and the molecular cluster 340 is a7-atom ionic cluster made of AgBr₂ or LaO₂. The ionic clusters 338 and340 are designed for V_(t) adjustment and high current flow in the “on”state for the respective p-channel and n-channel devices.

FIG. 9C shows a corresponding cross-sectional slice through the PFETdevice. In addition to the recessed gates, recessed source and draincontacts 318 are shown as embedded in the source and drain regions. Thesource and drain contacts 318 are in the form of metal quantum dots thatare insulated by a layer of neutral molecular clusters 342 and 344formed on the sidewalls and the bottom of the source and drain contacts318, at the interface with the source and drain regions 306 and 308.

The two embodiments shown in FIGS. 8 and 9A-9C provide examples ofdevices that can accommodate and benefit from the use of molecularcluster thin films. However, molecular clusters can be incorporated intodevices having many different geometries, and therefore the embodimentsshown are only exemplary and are not meant to be considered ascomprehensive.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A transistor formed on a substrate, thetransistor comprising: a source region; a drain region; a channel regionextending between the source region and the drain region; a metal gateregion including a metal gate that includes a conductive metal portion;contacts that couple the source region, the drain region, and the metalgate to a multi-layer metal interconnect structure; and a molecularcluster thin film contacting the conductive metal portion of the metalgate, the molecular cluster thin film comprising nanoscale molecularclusters including at least two bonded atoms that are different, thenanoscale molecular clusters having a selected atomic structure thatdetermines an electrical characteristic of the transistor, the molecularcluster thin film being between the conductive metal portion of themetal gate and the channel region.
 2. The transistor of claim 1 whereinthe molecular cluster thin film includes one or more of monomers,dimers, trimers, and tetramers.
 3. The transistor of claim 1 wherein themolecular cluster thin film is a work function metal film within themetal gate region, and the electrical characteristic is a thresholdvoltage at which the transistor is activated.
 4. The transistor of claim1 wherein the contacts comprise metal-insulator-semiconductor contactsto the source region and the drain region, themetal-insulator-semiconductor contacts including the molecular clusterthin film, and wherein the electrical characteristic is contactresistance.
 5. The transistor of claim 1 wherein the molecular clusterthin film includes one or more of silver bromide (Ag_(x)Br_(y)),lanthanum oxide (La_(x) O_(y)) and titanium oxide (Ti_(x)O_(y)).
 6. Thetransistor of claim 1 wherein the nanoscale molecular clusters have asize of less than 0.5 nm.
 7. The transistor of claim 1 wherein the metalgate and the contacts are recessed below a surface of the substrate. 8.The transistor of claim 1 wherein the molecular cluster thin film is anionic molecular thin film, and wherein the nanoscale molecular clustersare ionic nanoscale molecular clusters.
 9. The transistor of claim 8,further comprising a neutral molecular cluster thin film in the sourceregion and the drain region, wherein the neutral molecular thin filmcomprises neutral nanoscale molecular clusters.
 10. The transistor ofclaim 9 wherein the neutral nanoscale molecular clusters have a selectedspatial orientation.
 11. The transistor of claim 8 wherein the ionicnanoscale molecular clusters are positively charged.
 12. The transistorof claim 8 wherein the ionic nanoscale molecular clusters have aselected spatial orientation.
 13. An n-type semiconductor device,comprising: a silicon substrate having an oxide layer buried therein; anegatively-doped source region; a negatively-doped drain region; achannel through which current flows between the negatively-doped sourceregion and the negatively-doped drain region; a neutral molecularcluster thin film in the negatively-doped source region and thenegatively-doped drain region, the neutral molecular cluster thin filmcomprising neutral molecular clusters; a metal gate region comprising ametal gate capacitively coupled to the channel so as to control thecurrent, the metal gate including a conductive metal portion; an ionicmolecular cluster thin film contacting the conductive metal portion ofthe metal gate, the ionic molecular cluster thin film comprising ionicmolecular clusters, the molecular cluster thin film being between theconductive metal portion of the metal gate and the channel; andmetal-insulator-semiconductor contacts to the source region and thedrain region.
 14. The n-type semiconductor device of claim 13 whereinthe neutral molecular clusters have a selected spatial orientation. 15.The n-type semiconductor device of claim 13 wherein the ionic molecularclusters impart a tensile stress at a silicon/germanium interface in thechannel.
 16. A p-type semiconductor device, comprising: a siliconsubstrate having an oxide layer buried therein; a positively-dopedsource region; a positively-doped drain region; a channel through whichcurrent flows between the positively-doped source region and thepositively-doped drain region, the channel having a strained siliconinterface; a metal gate region comprising a metal gate capacitivelycoupled to the channel so as to control the current, the metal gateincluding a conductive metal portion and a dielectric portion; amolecular cluster thin film contacting the conductive metal portion ofthe metal gate region, the molecular cluster thin film being between theconductive metal portion and the dielectric portion of the first metalgate structure, the molecular cluster thin film comprising ionicclusters; and contacts to the positively-doped source region and thepositively-doped drain region.
 17. The p-type semiconductor device ofclaim 16 wherein the ionic clusters have a selected spatial orientation.18. The p-type semiconductor device of claim 16 wherein the ionicclusters impart a compressive stress at the strained silicon interfacein the channel.
 19. A device, comprising: a metal thin film, including aplurality of ionic clusters having a selected spatial orientation; asemiconductor substrate; a transistor on the semiconductor substrate,the transistor having a channel and a metal gate above the channel, themetal gate including a conductive metal material, the metal thin filmbeing in contact with the conductive metal material of the metal gate ofthe transistor, the metal thin film being between the conductive metalmaterial of the metal gate and the channel.
 20. The device of claim 19wherein a surface of the semiconductor substrate is electricallycharged.
 21. A device, comprising: a substrate; and a first transistoron the substrate, the first transistor including: a first source; afirst drain; a first metal gate structure that includes a conductivemetal portion and a dielectric portion; and a first molecular clusterthin film in contact with the conductive metal portion of the firstmetal gate structure, the first molecular cluster thin film beingbetween the conductive metal portion and the dielectric portion of thefirst metal gate structure.
 22. The device of claim 21 wherein the firsttransistor includes a channel that extends between the first source andfirst drain, the first metal gate structure being on the channel. 23.The device of claim 22 wherein the first molecular cluster thin film isin contact with a bottom surface and sides surfaces of the conductivemetal portion of the first metal gate structure.
 24. The device of claim21, further comprising: a second transistor on the substrate, the secondtransistor including: a second source; a source contact; a second drain;a drain contact; a second metal gate structure; a second molecularcluster thin film in contact with the source contact and is between thesource contact and the second source; and a third molecular cluster thinfilm in contact with the drain contact and is between the drain contactand the second drain.
 25. The device of claim 24 wherein the second andthird molecular cluster thin film include molecular clusters of the samestructure.
 26. The device of claim 24 wherein a structure of the firstmolecular cluster thin film is different from a structure of the secondmolecular cluster thin film.